Positive electrode active material for secondary batteries with nonaqueous electrolytic solution, process for the production of the active material, and secondary batteries with nonaqueous electrolytic solution

ABSTRACT

The present invention relates to positive electrode active substance particles for lithium ion batteries, comprising lithium manganate particles comprising Li and Mn as main components and having a cubic spinel structure (Fd-3m), wherein primary particles of the positive electrode active substance have a dodecahedral or higher-polyhedral shape in which none of crystal planes equivalent to the (111) plane are located adjacent to each other, and flat crystal planes are crossed with each other to form a clear ridge, and an average primary particle diameter of the primary particles is not less than 1 μm and not more than 20 μm. The positive electrode active substance particles according to the present invention are excellent in packing property, load characteristics and high-temperature stability.

TECHNICAL FIELD

The present invention relates to a non-aqueous electrolyte secondarybattery which comprises a positive electrode (cathode) active substancewith a well-controlled particle configuration to realize improvement inpacking property of the positive electrode active substance andhigh-temperature characteristics of the battery, and which has a longservice life and is excellent in load characteristics.

BACKGROUND ART

With the recent rapid development of portable and cordless electronicdevices such as audio-visual (AV) devices and personal computers, thereis an increasing demand for secondary batteries having a small size, alight weight and a high energy density as a power source for drivingthese electronic devices. Under these circumstances, lithium ionsecondary batteries having advantages such as a high charge/dischargevoltage and a large charge/discharge capacity have been noticed.

Hitherto, as positive electrode (cathode) active substances useful forhigh energy-type lithium ion secondary batteries exhibiting a 4 V-gradevoltage, there are generally known LiMn₂O₄ having a spinel structure,and LiCoO₂, LiCO_(1-x)Ni_(x)O₂ and LiNiO₂ having a layered rock-salttype structure, or the like. Among these active substances, LiCoO₂ ismore excellent because of a high voltage and a high capacity thereof,but has problems such as a high production cost due to a less amount ofa cobalt raw material supplied, and a poor environmental safety upondisposal of batteries obtained using the substance. In consequence,there have now been made earnest studies on lithium manganate having aspinel type structure (basic composition: LiMn₂O₄; this is hereinafterdefined in the same way) which is produced by using, as a raw material,manganese having a large supply amount, a low cost and a goodenvironmental compatibility. Further, although the layered rock-salttype structure has a two-dimensional diffusion path, the spinelstructure has a three-dimensional Li diffusion path. Therefore, it isexpected that the latter spinel structure is used as a positiveelectrode active substance in the applications requiring a largeelectric current, in particular, in the applications of a largesecondary batteries for automobiles.

As is known in the art, the lithium manganate particles may be obtainedby mixing a manganese compound and a lithium compound at a predeterminedratio and then calcining the resulting mixture in a temperature range of700 to 1000° C.

However, when the lithium manganate is highly enhanced incrystallizability in order to obtain a crystal structure suitable for anenhanced performance of the battery, the resulting lithium manganateparticles have an octahedral shape with a low packing rate as anautomorphic shape of the cubic spinel structure as shown in FIG. 7.Therefore, when using the lithium manganate particles having such anoctahedral structure as a positive electrode active substance forlithium ion secondary batteries, there tends to arise such a problemthat the obtained battery is deteriorated in capacity. In addition, thebattery tends to be deteriorated in charge/discharge cyclecharacteristics and storage characteristics under high-temperatureconditions. The reason therefor is considered to be that whencharge/discharge cycles are repeated, the crystal lattice is expandedand contracted owing to desorption and insertion behavior of lithiumions in the crystal structure to cause change in volume of the crystal,which results in occurrence of breakage of the crystal lattice,deteriorated current collecting property of the electrode or elution ofmanganese in an electrolyte solution.

At present, in the lithium ion secondary batteries using the lithiummanganate particles, it has been strongly required that the positiveelectrode active substance is packed in an electrode with a high packingdensity, the electrode formed from the positive electrode activesubstance has a low electric resistance, and the resulting batteries arefree from deterioration in charge/discharge capacity due to repeatedcharge/discharge cycles and improved in their characteristics, inparticular, under high-temperature conditions.

In order to improve the charge/discharge cycle characteristics of thebatteries under high-temperature conditions, it is necessary that thepositive electrode active substance used therein which comprises thelithium manganate particles has an excellent packing property and anappropriate particle size, and further is free from elution of manganesetherefrom. To meet these requirements, there have been proposed themethod of suitably controlling a particle size and a particle sizedistribution of the lithium manganate particles; the method of obtainingthe lithium manganate particles having a high crystallinity bycontrolling a calcination temperature thereof (Patent Document 1); themethod of adding different kinds of elements to the lithium manganateparticles to strengthen a bonding force between crystals thereof (PatentDocuments 2 to 4); the method of subjecting the lithium manganateparticles to surface treatment or adding additives thereto to suppresselution of manganese therefrom (Patent Documents 5 and 6); or the like.

Also, in Patent Document 7, there is described the method of reducing anelectric resistance of a positive electrode active substance byimproving a crystallizability of the lithium manganate particles andthereby obtaining particles having an octahedral shape or a generallyoctahedral shape.

Patent Document 1: Japanese Patent Application Laid-Open (KOAKI) No.2001-206722

Patent Document 2: Japanese Patent Application Laid-Open (KOAKI) No.2000-215892

Patent Document 3: Japanese Patent Application Laid-Open (KOAKI) No.2002-145617

Patent Document 4: Japanese Patent Application Laid-Open (KOAKI) No.2008-251390

Patent Document 5: Japanese Patent Application Laid-Open (KOAKI) No.2000-58055

Patent Document 6: Japanese Patent Application Laid-Open (KOAKI) No.2002-308628

Patent Document 7: Japanese Patent Application Laid-Open (KOAKI) No.2000-113889

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

At present, it has been strongly required to provide lithium manganateas a positive electrode active substance for a non-aqueous electrolytesecondary battery which is improved in output characteristics andhigh-temperature characteristics. However, the materials capable offully satisfying these requirements have not been obtained until now.

That is, even the techniques described in the above Patent Documents 1to 6 may fail to enhance a packing property and fully improve loadcharacteristics and high-temperature characteristics. In addition, theabove Patent Documents neither teach nor suggest that the crystals arecontrolled in their shape to enhance these properties.

Also, in the above Patent Document 7, it is described that the lithiummanganate particles are improved in crystallizability to obtain crystalparticles having an octahedral shape or a generally octahedral shape asan automorphic shape of the cubic spinel structure which results in areduced electric resistance of the positive electrode active substanceand an enhanced capacity retention rate thereof. However, PatentDocument 7 may fail to specify a packing property of the positiveelectrode active substance in lithium ion secondary batteries.

That is, the particles having an octahedral shape or a generallyoctahedral shape have a low packing property as compared to sphericalparticles having the same volume. In view of the packing property, it isconsidered to be more important that the particles are in the form ofpolyhedral particles constituted from a larger number of crystal planes,i.e., a higher-order polyhedron, in order to approach their shape tothat of spherical particles.

In addition, respective planes of the octahedron of the cubic spinelcrystals are constructed from the (111) plane and those crystal planesequivalent thereto.

On the other hand, a diffusion path of lithium ions in spinel manganesecrystals extends in the [110] direction and in the directions equivalentthereto. The charging and discharging of the lithium ion batteries areperformed by insertion and desorption of lithium ions in the positiveelectrode active substance. Therefore, it is considered to beadvantageous that the crystal plane extending in the direction closer tothat perpendicular to the [110] direction on which the diffusion path oflithium ions is located, is exposed onto a surface of the positiveelectrode active substance because the resistance against insertion anddesorption of the lithium ions becomes reduced. Assuming that the anglebetween the [110] direction or the direction equivalent thereto and aspecific crystal plane in the cubic spinel structure is 0, as isdetermined from the geometrical relationship therebetween, the angle θon the [111] plane is about 54.7°, the angle θ on the [221] plane isabout 74.2°, and the angle θ on the [110] plane is 90° (for example,refer to Cullity, “Elements of X-Ray Diffraction”, translated by Gentaromatsumura, Agunne, 6th Edition, p. 466). Therefore, in view offacilitated insertion and desorption of the lithium ions, it isconsidered to be more advantageous that the crystal plane appearing onthe surface of respective crystal particles of the positive electrodeactive substance is constituted of a less area of the {111} plane and abroader area of the {110} plane or the {221} plane.

In addition, it has been reported that one of the reasons fordeterioration of the lithium ion batteries resides in elution ofmanganese ions from the manganese spinel particles into an electrolytesolution owing to the disproportionation reaction as shown below whichmay occur in a high-temperature electrolyte solution.

2Mn³⁺(in spinel)→Me⁴⁺(in spinel)+Mn²⁺(in electrolyte)

It is considered that the elution of manganese occurs from portionshaving a large curvature. Therefore, it is considered that the structurehaving a sharp ridge (edge) or a sharp apex such as an octahedral shapeis more likely to suffer from the elution of Mn. In order to suppressthe elution of Mn, it is considered to be important that a curvature ofthe ridge formed by crossing crystal planes which constitute primaryparticles, i.e., an angle between the adjacent crystal planes, is formedinto a larger obtuse angle or into an apex having a less sharpness.

The reason why the cubic manganese spinel crystals are apt to have anoctahedral shape as an automorphic shape thereof which is constitutedfrom the (111) plane and the planes equivalent thereto, is considered tobe that the surface energy of the (111) plane or the planes equivalentthereto is smaller than that of the other crystal planes such as, forexample, (100) plane, (110) plane, (221) plane and planes equivalentthereto. For this reason, it is considered that the octahedral crystalsconstituted from the crystal planes equivalent to the (111) plane tendto be produced in order to minimize the surface energy of the crystalsas a whole. Therefore, it has been considered that if the surface energyof the crystal planes other than the crystal planes equivalent to the(111) plane is reduced, namely, if growth of the other crystal planes issuppressed, it is possible to obtain crystals having these crystalplanes.

Means for Solving the Problem

The above problems and technical tasks can be solved and accomplished bythe following aspects of the present invention.

That is, according to the present invention, there are provided positiveelectrode active substance particles for lithium ion batteries,comprising lithium manganate particles comprising Li and Mn as maincomponents and having a cubic spinel structure (space group: Fd-3m (No.227)),

primary particles of the positive electrode active substance having adodecahedral or higher-polyhedral shape in which none of crystal planesequivalent to the (111) plane are located adjacent to each other, andflat crystal planes are crossed with each other to form a clear ridge,and

an average primary particle diameter of the primary particles being notless than 1 μm and not more than 20 μm (Invention 1).

Also, according to the present invention, there are provided thepositive electrode active substance particles for lithium ion batteriesas described in the above Invention 1, wherein a ratio of Li to a sum ofMn and a substituting metal element [Li/(Mn substituting metal element)in which the substituting metal element is at least one metal elementother than Li and Mn with which an Mn (16d) site is substituted] in thepositive electrode active substance is not less than 0.5 (Invention 2).

In addition, according to the present invention, there is provided aprocess for producing the positive electrode active substance particlesas described in the above invention 1 or 2, comprising the steps ofmixing a manganese compound, a lithium compound and a crystal planegrowth inhibitor with each other; and calcining the resulting mixture ata temperature of 800 to 1050° C. (Invention 3).

Also, according to the present invention, there is provided the processfor producing the positive electrode active substance particles asdescribed in the above Invention 3, wherein the manganese compound is inthe form of secondary particles obtained by aggregating primaryparticles of Mn₃O₄ (trimanganese tetraoxide) having a generallyoctahedral shape (which is defined by any of an octahedral shape closeto a regular octahedral shape in which flat crystal planes are crossedwith each other to form a clear ridge; a near-octahedral shape in whicha portion at which four planes of an octahedron are crossed with eachother forms not a complete apex but a plane or a ridge; anear-octahedral shape in which a portion at which two planes of anoctahedron are crossed with each other forms not a complete ridge but aplane; and a near-octahedral shape which is formed by lacking a portionof these shapes) (Invention 4).

Also, according to the present invention, there is provided the processfor producing the positive electrode active substance particles asdescribed in the above Invention 3, wherein the crystal plane growthinhibitor is a phosphorus compound and/or an aluminum compound(Invention 5).

Further, according to the present invention, there is provided anon-aqueous electrolyte secondary battery comprising the positiveelectrode active substance particles as described in the above Invention1 or 2 (Invention 6).

Effect of the Invention

The positive electrode active substance particles according to thepresent invention are excellent in packing property as well as loadcharacteristics and high-temperature characteristics and, therefore, canbe suitably used as a positive electrode active substance fornon-aqueous electrolyte secondary batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a model view of a particle having a dodecahedral orhigher-polyhedral shape.

FIG. 2 is an electron micrograph showing positive electrode activesubstance particles obtained in Example 1.

FIG. 3 is an electron micrograph showing positive electrode activesubstance particles obtained in Example 3.

FIG. 4 is an electron micrograph showing positive electrode activesubstance particles according to the present invention.

FIG. 5 is an electron micrograph showing positive electrode activesubstance particles according to the present invention.

FIG. 6 is an electron micrograph showing manganese oxide particlesobtained in Example 1.

FIG. 7 is an electron micrograph showing lithium manganate having anoctahedral shape.

FIG. 8 is an electron micrograph showing positive electrode activesubstance particles obtained in Comparative Example 2.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

First, the positive electrode active substance particles according tothe present invention are described.

The positive electrode active substance particles according to thepresent invention comprises lithium manganate (stoichiometriccomposition: LiMn₂O₄) comprising Li and Mn as main components and havinga cubic spinel structure (Fd-3m (No. 227)). However, the positiveelectrode active substance of the present invention is not particularlylimited to those having the above stoichiometric composition, and mayalso include those substances in which anions are deficient orexcessive, or oxygen ions are deficient or excessive, as long as thecrystal structure can be maintained.

Meanwhile, in the positive electrode active substance particlesaccording to the present invention, a part of Mn may be substituted withthe other metal element, for example, one or more anions selected fromthe group consisting of Li, Fe, Mn, Ni, Mg, Zn, B, Al, Co, Cr, Si, Sn, Vand Sb, etc.

In the present invention, in particular, when using a phosphoruscompound and/or an aluminum compound as a crystal plane growthinhibitor, it is possible to obtain the positive electrode activesubstance particles having a desired shape. The content of thephosphorus component in the positive electrode active substanceparticles according to the present invention is preferably 0.0001 to0.05 in terms of a molar ratio of P based on Mn. The content of thealuminum component in the positive electrode active substance particlesaccording to the present invention is preferably 0.01 to 0.5 in terms ofa molar ratio of Al based on Mn.

In this case, in particular, when using the positive electrode activesubstance having a ratio of Li to a sum of Mn and the substituting metalelement [Li/(Mn+substituting metal element)] of not less than 0.5 amongthe above Li₂MnO₄ compositions, the resulting secondary battery can befurther lowered in internal resistance and can be enhanced in output ascompared to those using the positive electrode active substance havingthe above stoichiometric composition. The LiMn₂O₄ having a Li/Mn ratioof more than 0.5 include, for example, Li(Li_(x)Mn_(2-x))O₄ wherein x isan amount of the substituting metal element, which is obtained bysubstituting a part of Mn with Li as the substituting metal element. Theratio [Li/(Mn substituting metal element)] is preferably 0.5 to 0.65.

The primary particles of the positive electrode active substanceparticles according to the present invention have a dodecahedral orhigher-polyhedral shape in which none of the crystal planes equivalentto the (111) plane are located adjacent to each other, and flat crystalplanes are crossed with each other to form a clear ridge.

In general, the positive electrode active substance may be molded into aplate shape by a press-molding method, or may be molded by applying aslurry prepared by adding the positive electrode active substance and aconductive assistant to a solvent in which a binder is dissolved, onto asurface of a metal foil. In this case, as the amount of the positiveelectrode active substance contained per a unit volume of the resultingmolded product becomes larger, the capacity of the obtained positiveelectrode can be increased. Therefore, it is desirable to increase apacking density of the positive electrode active substance.

In view of the closest packing structure, when one particle is definedas a rigid sphere, the packing rate of the positive electrode activesubstance particles is 74%. The packing rate of particles having aregular octahedral shape as an automorphic shape of the lithiummanganate is about 67% when calculated in the same manner as above.Therefore, it is considered that the packing property of the positiveelectrode active substance particles can be further increased by formingthe primary particles thereof into a polyhedral shape much closer to asphere.

The positive electrode active substance particles according to thepresent invention have neither an octahedral shape as an automorphicshape of the cubic spinel structure nor any shapes similar thereto. Inthe particles having an octahedral shape as an automorphic shape of thelithium manganate, the rate of growth of the {111} plane is slower thanthose of the other crystal planes during a crystal growth of theparticles, so that the octahedral particles are constituted from the{111} plane. Therefore, in order to well control a shape of theparticles, crystal growth of the crystal planes other than the {111}plane is suppressed, whereby it is possible to allow the crystal planeswhich are usually dissipated during the crystal growth to remain on theparticles.

In the particles having an octahedral shape as an automorphic shape ofthe lithium manganate, the angle between the crystal planes equivalentto the (111) plane is 109.15°. In the polyhedral particles having adodecahedral or higher-polyhedral shape according to the presentinvention in which crystal growth of the (110) plane, the (111) planeand the crystal planes equivalent to these planes is suppressed, and thecrystal planes equivalent to the (111) plane are prevented from beinglocated adjacent to each other, the angle between any crystal planesthereof is larger than 109.15°.

In this regard, an example of a model of the polyhedral particles isshown in FIG. 1. In addition, in FIG. 2 to FIG. 5, there are shownvarious shapes of the positive electrode active substance particlesaccording to the present invention. The respective polyhedral particlesas shown therein are particles having 12 or more planes in which crystalgrowth of the (100) plane, the (110) plane, the (221) plane and theplanes equivalent to these planes in the octahedron as an automorphicshape of the lithium manganate is suppressed. The polyhedral particlesshown in FIG. 1 is only illustrative, and may also comprise anypolyhedral particles including crystal planes other than the {111}plane, the {221} plane, the {110} plane and the {100} plane.

Also, it is expected that such polyhedral particles have the effect ofenhancing an efficiency of insertion and desorption of lithium ionstherein. When noting the Li atoms in the manganese spinel crystalstructure, it is considered that the insertion and desorption of Li ionsare more efficiently caused in the <110> direction. Therefore, it issuggested that the [110] plane perpendicular to the <110> direction isthe plane having the highest Li ionic conductivity. For this reason, itis desirable that the clear {110} plane remains in a state surrounded byridges by controlling growth of the crystal plane.

The dodecahedral or higher-polyhedral particles according to the presentinvention may also include those particles formed by allowing primaryparticles to cross with each other, those particles in which crystalplanes are commonly shared among a plurality of primary particles, orone primary particle is grown from a part of a surface of the otherprimary particle, those particles which are formed by lacking a portionof these particle shapes, and those particles produced by sharingcrystal planes among primary particles in a complicated manner.

The positive electrode active substance particles according to thepresent invention have the particle shape as defined in the aboveInvention 1. However, the positive electrode active substance particlesmay also comprise primary particles having the other shape such as anoctahedral shape and a granular shape as long as the secondary batteryproduced using the particles is excellent in capacity recovery rate,high-temperature cycle capacity and rate characteristic. Morespecifically, the definition that the “primary particles have adodecahedral or higher-polyhedral shape in which none of the crystalplanes equivalent to the (111) plane are located adjacent to each other,and flat crystal planes are crossed with each other to form clear ridge”according to the present invention means that the content of thepolyhedral particles as defined above in the whole positive electrodeactive substance particles is not less than 75% and preferably not lessthan 95%. Meanwhile, the content of the polyhedral particles as usedabove means the proportion of the number of the particles which arerecognized to have the above polyhedral shape relative to the number ofthe whole particles observed on the below-mentioned scanning electronmicrograph.

The positive electrode active substance particles according to thepresent invention have an average primary particle diameter of not lessthan 1 μm and not more than 20 μm, preferably 1.2 to 10 μm and morepreferably 1.3 to 8 μm.

The average secondary particle diameter (D50) of the positive electrodeactive substance particles according to the present invention isadjusted such that the ratio of the average secondary particle diameter(D50) of the positive electrode active substance particles to an averagesecondary particle diameter (D50) of the manganese compound as aprecursor thereof is not more than 1.35. When the ratio of the averagesecondary particle diameter (D50) of the positive electrode activesubstance particles to that of the precursor particles is more than1.35, the primary particles of the positive electrode active substanceparticles tend to be excessively grown, so that the resulting secondarybattery tends to be deteriorated in output. Further, the primaryparticles tend to be aggregated together, so that elution of Mn tends tobe promoted from the aggregated portions, resulting in deterioratedhigh-temperature characteristics of the resulting secondary battery. Theratio of the average secondary particle diameter (D50) of the positiveelectrode active substance particles to that of the precursor particlesis preferably not more than 1.33 and more preferably not more than 1.30.

The positive electrode active substance particles according to thepresent invention have a BET specific surface area of 0.3 to 1.5 m²/g.When the BET specific surface area of the positive electrode activesubstance particles is less than 0.3 m²/g, the resulting particles tendto suffer from promoted aggregation therebetween and tends to betherefore deteriorated in stability. When the BET specific surface areaof the positive electrode active substance particles is more than 1.5m²/g, the resulting particles tend to be unstable by themselves. The BETspecific surface area of the positive electrode active substanceparticles is preferably 0.35 to 1.3 m²/g and more preferably 0.4 to 1.2m²/g.

The positive electrode active substance particles according to thepresent invention preferably have a packing density (when tapped 500times) of not less than 1.8 g/cm³. When the packing density of thepositive electrode active substance particles is less than 1.8 g/cm³,the electrode obtained using the positive electrode active substanceparticles tends to be deteriorated in packing property, so that it maybe difficult to attain a high capacity of the resulting battery. Whenthe packing density of the positive electrode active substance particlesis more preferably not less than 1.85 g/cm³.

The positive electrode active substance particles according to thepresent invention preferably have a compressed density of not less than2.85 g/cm³ when applying a pressure of 3 ton/cm³ thereto. When thecompressed density of the positive electrode active substance particlesis less than 2.85 g/cm³, the obtained particles tend to be deterioratedin packing property, so that it may be difficult to attain a highcapacity of the resulting battery. The compressed density of thepositive electrode active substance particles is more preferably notless than 2.90 g/cm³.

The positive electrode active substance particles according to thepresent invention have a lattice constant of 0.8185 to 0.822 nm asmeasured by a Rietveld method.

The primary particles of the positive electrode active substanceparticles according to the present invention are constituted fromsubstantially a single crystal. When the primary particles of thepositive electrode active substance particles are constituted of apolycrystal, a large number of lattice-unconformity planes acting as aresistance component against the insertion and desorption of Li tend tobe present in the crystals, so that it may be difficult to allow theresulting battery to generate a sufficient output.

Next, the method of producing a positive electrode using the positiveelectrode active substance particles according to the present inventionis described.

When producing the positive electrode using the positive electrodeactive substance particles according to the present invention, aconducting agent and a binder are added to and mixed with the positiveelectrode active substance particles by an ordinary method. Examples ofthe preferred conducting agent include acetylene black, carbon black andgraphite. Examples of the preferred binder includepolytetrafluoroethylene and polyvinylidene fluoride.

The secondary battery produced by using the positive electrode activesubstance particles according to the present invention comprises theabove positive electrode, a negative electrode and an electrolyte.

Examples of a negative electrode active substance for the negativeelectrode include metallic lithium, lithium/aluminum alloys, lithium/tinalloys, and graphite or black lead.

Also, as a solvent for the electrolyte solution, there may be usedcombination of ethylene carbonate and diethyl carbonate, as well as anorganic solvent comprising at least one compound selected from the groupconsisting of carbonates such as propylene carbonate and dimethylcarbonate, and ethers such as dimethoxyethane.

Further, as the electrolyte, there may be used a solution prepared bydissolving, in addition to lithium phosphate hexafluoride, at least onelithium salt selected from the group consisting of lithium perchlorate,lithium borate tetrafluoride and the like in the above solvent.

In addition, the battery characteristics of the positive electrodeactive substance particles according to the present invention areevaluated as follows. That is, the evaluation for the batterycharacteristics is carried out using a non-aqueous electrolyte secondarybattery of a CR2032 type which is produced from the positive electrodeactive substance particles, a non-aqueous electrolyte solution (mixedsolution comprising EC and DEC; mixing ratio of EC:DEC=3:7) to which 1mol/L LiPF₆ is added, and a 500 μm-thick Li foil as a negativeelectrode.

The secondary battery produced using the positive electrode activesubstance particles according to the present invention has an initialdischarge capacity of 80 to 120 mAh/g. When the initial dischargecapacity is less than 80 mAh/g, the secondary battery tends to be hardlyused in practice owing to a low output therefrom. When the initialdischarge capacity is more than 120 mAh/g, the secondary battery tendsto hardly maintain a sufficient stability. The initial dischargecapacity of the secondary battery is preferably 90 to 115 mAh/g.

The secondary battery produced using the positive electrode activesubstance particles according to the present invention preferably has ahigh-temperature cycle capacity retention rate of not less than 90%. Thehigh-temperature cycle capacity retention rate of the secondary batteryis more preferably not less than 93% and still more preferably not lessthan 95%.

The secondary battery produced using the positive electrode activesubstance particles according to the present invention preferably has acapacity recovery rate of not less than 95% and more preferably not lessthan 97%.

The secondary battery produced using the positive electrode activesubstance particles according to the present invention preferably has arate characteristic of not less than 90%, more preferably not less than93% and still more preferably not less than 95%.

Next, the process for producing the positive electrode active substanceparticles according to the present invention is described.

The positive electrode active substance particles according to thepresent invention are produced by mixing a manganese compound, a lithiumcompound and a crystal plane growth inhibitor, if required, togetherwith a substituting metal element compound, and then calcining theresulting mixture in a temperature range of not lower than 800° C., andpreferably 800 to 1050° C.

Examples of the manganese compound used in the present invention includetrimanganese tetraoxide (Mn₃O₄), manganese dioxide (γ-MnO₂, β-MnO₂),dimanganese trioxide, manganese carbonate, manganese chloride andmanganese sulfate. Among these manganese compound, trimanganesetetraoxide (Mn₃O₄) is especially preferred. The trimanganese tetraoxide(Mn₃O₄) preferably has an average primary particle diameter of 0.5 to 20μm and more preferably 1 to 10 μm and a BET specific surface area of 0.5to 15 m²/g, and the shape of the trimanganese tetraoxide (Mn₃O₄) ispreferably an octahedral shape or a generally octahedral shape. The“generally octahedral shape” as used herein means any of an octahedralshape close to a regular octahedral shape in which flat crystal planesare crossed with each other to form a clear ridge; a near-octahedralshape in which a portion at which four planes of an octahedron arecrossed with each other forms not a complete apex but a plane or aridge; a near-octahedral shape in which a portion at which two planes ofan octahedron are crossed with each other forms not a complete ridge buta plane; and a near-octahedral shape which is formed by lacking aportion of these shapes. In addition, the particles having an octahedralshape or a generally octahedral shape may also include such particles inwhich crystal planes are shared among primary particles, or a primaryparticle crystal is grown form a part of a surface of the other primaryparticle. FIG. 6 shows an electron micrograph of trimanganese tetraoxideparticles having an octahedral shape.

The substituting metal element used in the present invention includes atleast one metal element other than Li and Mn with which the Mn (16d)site can be substituted. Any metal elements may be used as thesubstituting metal element as long as they reduce an amount of trivalentmanganese (Mn³⁺) in the manganese spinel positive electrode activesubstance to control a charge/discharge capacity of the resultingbattery and thereby improve charge/discharge cycle characteristics andhigh-temperature characteristics thereof. The substituting metal elementis preferably Al or Mg. These substituting metal elements are preferablyuniformly dispersed within the respective positive electrode activesubstance particles according to the present invention. When thesubstituting metal elements are unevenly localized in the respectiveparticles, the non-aqueous electrolyte secondary battery produced usingthe positive electrode active substance particles tends to bedeteriorated in stability.

As the crystal plane growth inhibitor used in the present invention,there may be mentioned a phosphorus compound and an aluminum compound.Examples of the phosphorus compound include ammonium dihydrogenphosphate (NH₄H₂PO₄), lithium phosphate, calcium phosphate, trisodiumphosphate and sodium dihydrogen phosphate. Examples of the aluminumcompound include aluminum hydroxide (Al(OH)₂), aluminum chloride andaluminum sulfate. The phosphorus compound may be used in combinationwith the aluminum compound. Among these compounds, preferred arephosphorus compounds, and especially preferred is ammonium dihydrogenphosphate (NH₄H₂PO₄). The phosphorus compound preferably has an averagesecondary particle diameter (D50) of 1 to 50 μm.

The amount of the phosphorus compound added may be 0.01 to 0.7 mol % interms of P based on Mn. When the amount of the phosphorus compound addedis less than 0.01 mol % based on Mn, no sufficient effect by theaddition of the phosphorus compound tends to be attained. When theamount of the phosphorus compound added is more than 0.7 mol % based onMn, an excessive amount of P added tends to form a compound which willact as a resistance component on the surface of the resulting positiveelectrode active substance particles. The amount of the phosphoruscompound added is preferably 0.02 to 0.5 mol % and more preferably 0.02to 0.3 mold.

In the present invention, Al as the substituting metal element also hasan effect of the crystal plane growth inhibitor. The positive electrodeactive substance comprising Al may be produced by the method of mixingthe manganese compound, the lithium compound and the aluminum compoundwith each other at a predetermined mixing ratio and then calcining theresulting mixture in a temperature range of 800 to 1050° C., the methodof previously coating the surface of respective particles of themanganese compound with the aluminum compound, mixing the resultingcoated particles with the lithium compound, and then calcining theresulting mixture in the above temperature range, or the like.

In the present invention, when the positive electrode active substanceis produced using only the aluminum compound as the crystal plane growthinhibitor, the average secondary particle diameter of the manganesecompound as one of the starting materials is preferably as small aspossible, and is, for example, 1.0 to 2.5 μm.

<Function>

In accordance with the present invention, the positive electrode activesubstance particles having the above properties can be produced byuniformly mixing a manganese compound, a lithium compound and a crystalplane growth inhibitor with each other and then calcining the resultingmixture in air at a temperature of 800 to 1050° C.

As a result, it is considered that the secondary battery produced usingthe positive electrode active substance particles according to thepresent invention can be enhanced in electrode packing property orhigh-temperature characteristics such as effect of preventing elution ofMn, and at the same time can be improved in output characteristics.

EXAMPLES

Typical examples of the present invention are described in more detailbelow.

The average primary particle diameter of the particles was expressed byan average value of diameters of the particles which were observed usinga scanning electron microscope “SEM-EDX” equipped with an energydisperse type X-ray analyzer (manufactured by Hitachi High-TechnologiesCorp.) and read out from a SEM image thereof.

The average secondary particle diameter (D₅₀) of the particles wasdetermined from a volume median particle diameter as measured by a wetlaser method using a laser type particle size distribution measuringapparatus “MICROTRACK HRA” manufactured by Nikkiso Co., Ltd.

The BET specific surface area of the particles was measured as follows.That is, a sample was dried and deaerated under a nitrogen gasatmosphere at 120° C. for 45 min, and the BET specific surface area ofthe thus treated sample was measured using “MONOSORB” manufactured byYuasa Ionics Inc.

The packing density of the positive electrode active substance particleswas measured as follows. That is, 40 g of the particle were weighed andcharged into a 50 cm³ measuring cylinder, and then tapped 500 timesusing a “TAP DENSER” manufactured by Seishin Enterprises Co., Ltd., toread out a volume of the tapped particles and calculate a packingdensity of the particles therefrom.

The compressed density of the positive electrode active substanceparticles was determined as follows. That is, 1 g of the particles wascharged into a φ10 metal mold, and compressed therein while increasing apressure applied thereto by each 0.5 t/cm² in the range of from 1 to 4t/cm². The value of a density of the particles as measured upon applyinga pressure of 3 t/cm² thereto was used as the compressed density.

The X-ray diffraction of the sample was measured using “RAD-IIA”manufactured by Rigaku Co., Ltd.

The lattice constant was calculated from the results of the above powderX-ray diffraction by a Rietveld method.

Whether the crystal structure of the particles was a single crystal ornot was confirmed by observing an oriented plane of a section of theparticles by EBSD analysis.

<Evaluation of Battery Characteristics of Positive Electrode ActiveSubstance>

The coin cell of a CR2032 type was produced by using the positiveelectrode active substance particles according to the present invention,and the battery characteristics of the thus produced coin cell wereevaluated. First, 92% by weight of an Li—Mn composite oxide as apositive electrode active substance, 2.5% by weight of acetylene blackas a conducting material, and 3% by weight of polyvinylidene fluoridedissolved in N-methylpyrrolidone as a binder, were mixed with eachother, and the resulting mixture was applied onto an Al metal foil andthen dried at 110° C. The thus obtained sheets were each blanked into 16mmφ and then compression-bonded together by applying a pressure of 1.7t/cm² thereto, thereby producing an electrode having a thickness of 50μm and using the thus produced electrode as a positive electrode. A 500μm-thick metallic lithium blanked into 16 mmφ was used as a negativeelectrode, and a solution prepared by mixing EC and DEC with each otherat a volume ratio of 3:7 in which 1 mol/L of LiPF₆ was dissolved, wasused as an electrolyte solution, thereby producing the coin cell of aCR2032 type.

The capacity recovery rate of the thus produced coin cell of a CR2032type was evaluated in the following manner. That is, the coin cell wassubjected to CC-CV charging at a current density of 0.1 C until reaching4.3 V, and then discharged at 0.1 C until reaching 3.0 V. The dischargecapacity of the coin cell upon the above charge/discharge cycle wasexpressed by “a”. Next, the coin cell was charged until reaching acharge depth of 50% (SOC: 50%). Thereafter, the coin cell was allowed tostand at 60° C. for one week, taken out, and then discharged at 0.1 Cuntil reaching 3.0 V. Then, the coin cell was subjected to charging anddischarging at 0.1 C to measure a discharge capacity (d) of the coincell. The capacity recovery rate of the coin cell was calculated fromthe formula: 100×d/a.

The high-temperature cycle capacity retention rate of the above coincell of a CR2032 type was evaluated as follows. That is, the coin cellwas subjected to charging and discharging at 1 C in the range of from3.0 to 4.3 V (the discharge capacity obtained thereupon was expressed by“a”), and then repeatedly subjected to twenty nine (29) charging anddischarging cycles at 1 C in the range of from 3.0 to 4.3 V (in whichCC-CV charging and CC-CC discharging were respectively repeated), andthe discharge capacity at the 29th cycle was expressed by “b”. The cyclecapacity retention rate of the coin cell was calculated from theformula: b/a×100(%).

Further, the rate characteristic of the above coin cell of a CR2032 typewas evaluated as follows. That is, the coin cell was subjected tocharging and discharging cycles at 25° C. in a voltage range of 3.0 to4.3 V in which the charging was conducted at a current density of 0.1 C(CC-CV), whereas the discharging was conducted at a current density of0.1 C, 0.2 C, 0.5 C, 1.0 C, 2.0 C and 5.0 C. At this time, the value ofa discharge capacity at 0.1 C was expressed by “e”, and the value of adischarge capacity at 5.0 C was expressed by “f”. The ratecharacteristic of the coin cell was calculated from the formula:f/e×100(%).

Example 1 Production of Positive Electrode Active Substance Particles

Under a nitrogen flow, 0.5 mol of manganese sulfate was added to 3.5 molof sodium hydroxide to prepare a reaction solution having a total volumeof 1 L. Manganese hydroxide thus produced was aged at 90° C. for 1 hr.After completion of the aging, air was passed through the reactionsolution to oxidize manganese hydroxide at 90° C., and the resultingproduct was washed with water and then dried, thereby obtainingmanganese oxide particles.

The thus obtained manganese oxide particles were Mn₃O₄ and had anoctahedral particle shape as shown in FIG. 6. In addition, the manganeseoxide particles had an average secondary particle diameter of 5.2 μm anda BET specific surface area of 0.6 m²/g.

The above manganese oxide (Mn₃O₄), lithium carbonate (Li₂CO₃) andaluminum hydroxide (Al(OH)₃) were weighed such that a molar ratio ofLi:Mn:Al was 1.073:1.830:0.096, and further ammonium dihydrogenphosphate (NH₄H₂PO₄) was weighed in an amount of 0.05 mold in terms of Pbased on Mn, and the thus weighed compounds were mixed with each other,and then calcined in atmospheric air at 960° C. for 3 hr to therebyobtain lithium manganate particles.

As a result of XRT diffraction analysis (using “RAD-IIA” manufactured byRigaku Corp.), it was confirmed that the thus obtained lithium manganateparticles comprised no different phases. In addition, as a result ofobserving an SEM image of the lithium manganate particles (using an SEMmanufactured by Hitachi High-Technologies Corp.), it was confirmed thatthe particles had a polyhedral shape as shown in FIG. 2. That is, theprimary particles of the lithium manganate particles exhibited neitheran octahedral shape nor a shape close thereto, and had such a polyhedralshape which was constituted from flat crystal planes including the (111)plane, (221) plane, (110) plane, (100) plane and crystal planesequivalent to these planes, and in which none of the crystal planesequivalent to the (111) plane were located adjacent to each other, theflat crystal planes were crossed with each other to form a clear ridge,and the angle between any adjacent ones of the crystal planes was anobtuse angle larger than 109.15° which was an angle between the crystalplanes equivalent to the (111) plane when expressed as an obtuse angle.The proportion of the number of the above polyhedral particles relativeto the number of the whole lithium manganate particles was about 98%.

The resulting lithium manganate particles had an average primaryparticle diameter of 5 μm and an average secondary particle diameter(D50) of 6.2 μm, and the ratio of the above average secondary particlediameter (D50) of the lithium manganate particles to an averagesecondary particle diameter (D50) of a precursor thereof was 1.19.Further, the lithium manganate particles had a BET specific surface areaof 0.74 m²/g, a packing density of 1.91 g/cm³ and a compressed densityof 2.96 g/cm³.

The coin type battery produced by using a positive electrode activesubstance comprising the thus obtained lithium manganate particles hadan initial discharge capacity of 105 mAh/g, a capacity recovery rate of98%, a high-temperature cycle capacity retention rate of 97% and a ratecharacteristic of 96%.

Comparative Example 1

The same procedure as defined in Example 1 was conducted except that MgOwas used as the substituting metal element compound, and the amounts ofthe respective components added and the calcination temperature werechanged, thereby obtaining a positive electrode active substancecomprising lithium manganate particles. As a result, it was confirmedthat the primary particles of the thus obtained lithium manganateparticles had an octahedral shape, and the proportion of the number ofthe above polyhedral particles to the number of the whole lithiummanganate particles was about 70%.

Examples 2 and 3 and Comparative Example 2

The same procedure as defined in Example 1 was conducted except that thesubstituting metal elements used, the kinds and amounts of therespective additive element compounds and the calcination temperaturewere changed variously, thereby obtaining positive electrode activesubstances comprising lithium manganate particles. The productionconditions used in the Examples and Comparative Example are shown inTable 1, and various properties of the thus obtained lithium manganateparticles are shown in Table 2. As a result, it was confirmed that eventhe primary particles of the lithium manganate particles thus obtainedin Examples 2 and 3 had the same polyhedral shape as those obtained inExample 1. In FIG. 3, there is shown an electron micrograph of thelithium manganate particles obtained in Example 3. It was confirmed thatthe proportion of the number of the polyhedral particles to the numberof the whole lithium manganate particles was about 97%. Also, in FIG. 8,there is shown an electron micrograph of the lithium manganate particlesobtained in Comparative Example 2. As shown in FIG. 8, it was confirmedthat the primary particles of the lithium manganate particles obtainedin Comparative Example 2 had a rounded shape, and the proportion of thenumber of the rounded polyhedral particles to the number of the wholelithium manganate particles was about 20%.

TABLE 1 Precursor Mixing Average Substituting secondary metals andExamples Kind of Mn particle additive elements, and Comp. compounddiameter and ratios to Mn Examples (—) (μm) (—) (—) Example 1 Mn₃O₄ 5.2Al/P  0.05/0.0005 Example 2 Mn₃O₄ 5.2 Al/P 0.05/0.001 Example 3 Mn₃O₄2.4 Al 0.05  Comp. Mn₃O₄ 5.2 Mg 0.025 Example 1 Comp. Mn₃O₄ 5.2 Al/B0.05/0.015 Example 2 Mixing Kind of Kind of Examples Li/(Mn +substituting additive and Comp. substituting metal compound elementExamples metal) (—) (—) compound (—) Example 1 0.556 Al(OH)₃ NH₄H₂PO₄Example 2 0.556 Al(OH)₃ NH₄H₂PO₄ Example 3 0.556 Al(OH)₃ — Comp. 0.545MgO — Example 1 Comp. 0.556 Al(OH)₃ H₃BO₃ Example 2 Examples Calcinationconditions and Comp. Temperature in air Time Examples (° C.) (hr)Example 1 960 3 Example 2 960 3 Example 3 910 3 Comp. 870 3 Example 1Comp. 960 3 Example 2

TABLE 2 Precursor Examples Average secondary and Comp. Composition ofpositive particle diameter Examples electrode active substance (μm)Example 1 Li_(1.072)Mn_(1.828)Al_(0.1)O₄ + 0.00092P 5.2 Example 2Li_(1.072)Mn_(1.828)Al_(0.1)O₄ + 0.00183P 5.2 Example 3Li_(1.072)Mn_(1.828)Al_(0.1)O₅ 2.4 Comp. Li_(1.065)Mn_(1.905)Mg_(0.05)O₄5.0 Example 1 Comp. Li_(1.072)Mn_(1.828)Al_(0.1)O₄ + 0.0274B 4.8 Example2 Properties of particles according to the present invention Ratio ofaverage Average Average secondary primary secondary particle Examplesparticle particle diameter to and Comp. diameter diameter (D50) that ofExamples (μm) (μm) precursor (—) Example 1 5 6.2 1.19 Example 2 5 6.51.24 Example 3 1.4 4.8 1.99 Comp. 5 7.0 1.40 Example 1 Comp. 5.2 9.62.00 Example 2 Properties of particles according to the presentinvention Packing density Examples Specific (tapped 500 Compressed andComp. surface area times) density (3 t) Examples (m²/g) (g/cm³) (g/cm³)Example 1 0.74 1.91 2.96 Example 2 0.59 1.88 2.92 Example 3 1.08 1.402.66 Comp. 0.58 1.78 2.81 Example 1 Comp. 0.43 2.00 2.79 Example 2

Also, the evaluation results of battery characteristics of the CR2032type coin cells produced by using the positive electrode activesubstance particles according to the present invention are shown inTable 3.

TABLE 3 Battery characteristics Examples Initial and Comp. Compositionof positive discharge Examples electrode active substance capacity(mAh/g) Example 1 Li_(1.072)Mn_(1.828)Al_(0.1)O₄ + 0.00092P 105 Example2 Li_(1.072)Mn_(1.828)Al_(0.1)O₄ + 0.00183P 105 Example 3Li_(1.072)Mn_(1.828)Al_(0.1)O₄ 103 Comp. Li_(1.065)Mn_(1.905)Mg_(0.03)O₄109 Example 1 Comp. Li_(1.072)Mn_(1.828)Al_(0.1)O₄ + 0.0274B 108 Example2 Battery characteristics High- Rate temperature characteristic ExamplesCapacity cycle capacity (5 C/0.1 C) × and Comp. recovery rate retentionrate 100 Examples (%) (%) (%) Example 1 98 97 96 Example 2 98 97 96Example 3 99 92 98 Comp. 92 86 83 Example 1 Comp. 93 88 88 Example 2

INDUSTRIAL APPLICABILITY

The positive electrode active substance particles according to thepresent invention in which primary particles of the positive electrodeactive substance are well controlled in a crystal shape thereof, areexcellent in packing property as well as load characteristics and ahigh-temperature stability and, therefore, can be suitably used as apositive electrode active substance for secondary batteries.

1. Positive electrode active substance particles for lithium ionbatteries, comprising lithium manganate particles comprising Li and Mnas main components and having a cubic spinel structure (space group:Fd-3m (No. 227)), primary particles of the positive electrode activesubstance having a dodecahedral or higher-polyhedral shape in which noneof crystal planes equivalent to the (111) plane are located adjacent toeach other, and flat crystal planes are crossed with each other to forma clear ridge, and an average primary particle diameter of the primaryparticles being not less than 1 μm and not more than 20 μm.
 2. Positiveelectrode active substance particles for lithium ion batteries accordingto claim 1, wherein a ratio of Li to a sum of Mn and a substitutingmetal element [Li/(Mn+substituting metal element) in which thesubstituting metal element is at least one metal element other than Liand Mn with which an Mn (16d) site is substituted] in the positiveelectrode active substance is not less than 0.5.
 3. A process forproducing the positive electrode active substance particles as definedin claim 1, comprising the steps of mixing a manganese compound, alithium compound and a crystal plane growth inhibitor with each other;and calcining the resulting mixture at a temperature of 800 to 1050° C.4. A process for producing the positive electrode active substanceparticles according to claim 3, wherein the manganese compound is in theform of secondary particles obtained by aggregating primary particles ofMn₃O₄ (trimanganese tetraoxide) having a generally octahedral shapewhich is defined by any of an octahedral shape close to a regularoctahedral shape in which flat crystal planes are crossed with eachother to form a clear ridge; a near-octahedral shape in which a portionat which four planes of an octahedron are crossed with each other formsnot a complete apex but a plane or a ridge; a near-octahedral shape inwhich a portion at which two planes of an octahedron are crossed witheach other forms not a complete ridge but a plane; and a near-octahedralshape which is formed by lacking a portion of these shapes.
 5. A processfor producing the positive electrode active substance particlesaccording to claim 3, wherein the crystal plane growth inhibitor is aphosphorus compound and/or an aluminum compound.
 6. A non-aqueouselectrolyte secondary battery comprising the positive electrode activesubstance particles as defined in claim 1.